I wouldn't worry about a decoction. Save that for a German Lager (if you want to do one). You are only doing a partial mash, more like a mini mash so it would not work out nor would it be worth it regardless. You are doing such a small mash, that even mash temp won't play a big factor. But I would shoot for mashing in the mid 150's if you do so.
If you are looking for nutty character, Brown Malt and Pale Chocolate could help that in a Porter. But also malts like Biscuit or Victory will add that same character. Brown Malt, Biscuit or Victory all have to be mashed as well. Are you looking to brew a Historical style Porter by using the Sugar? Are you looking for something dryer, or something fuller, sweeter?